Microporous pvdf films

ABSTRACT

Shaped microporous articles are produced from polyvinylidene fluoride (PVDF) and nucleating agents using thermally induced phase separation (TIPS) processes. The shaped microporous article is oriented in at least one direction at a stretch ratio of at least approximately 1.1 to 1.0. The shaped article may also comprise a diluent, glyceryl triacetate. The shaped microporous article may also have the micropores filled with a sufficient quantity of ion conducting electrolyte to allow the membrane to function as an ion conductive membrane. The method of making a microporous article comprises the steps of melt blending polyvinylidene fluoride, nucleating agent and glyceryl triacetate; forming a shaped article of the mixture; cooling the shaped article to cause crystallization of the polyvinylidene fluoride and phase separation of the polyvinylidene fluoride and glyceryl triacetate; and stretching the shaped article in at least one direction at a stretch ratio of at least approximately 1.1 to 1.0.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/661,908,filed Sep. 12, 2003, now allowed, the disclosure of which isincorporated by reference in its entirely herein.

FIELD OF THE INVENTION

The present invention relates generally to microporous films. Inparticular, the present invention relates to microporous films formedfrom polyvinylidene fluoride and methods for making the same utilizingglyceryl triacetate and nucleating agents.

BACKGROUND OF THE INVENTION

Microporous films have a structure that enables fluids and/or gases toflow through. The effective pore size is at least several times the meanfree path of the flowing molecules, namely, from several micrometers anddown to about 100 angstroms. Sheets of the microporous films aregenerally opaque, even when made from an originally transparentmaterial, because the surfaces and internal structures scatter visiblelight.

Microporous films have been utilized in a wide variety of applications,such as filtration of solids, ultrafiltration of colloids, diffusionbarriers, and in cloth laminates. Additional applications include:filter cleaning antibiotics, beer, oils, and bacterial broths; analysisof air, microbiological samples, intravenous fluids and vaccines.Microporous films are also utilized in the preparation of surgicaldressings, bandages, and in other fluid transmissive medicalapplications.

Ion conductive membranes (ICMs) are also being developed frommicroporous films. Ion conductive membranes have found application inmembrane electrode assemblies (MEAs) as solid electrolytes. One specificexample application of an MEA is a hydrogen/oxygen fuel cell. The ICM islocated between the cathode and anode in the MEA, and transports protonsfrom near the catalyst at the hydrogen electrode to the oxygen electrodethereby allowing the current to be drawn from the MEA. The ICMs areparticularly advantageous in these applications as they replace acidicliquid electrolytes, such as are used in phosphoric acid fuel cells,which are very hazardous.

Ion conductive membranes are also used in chloroalkali applications toseparate brine mixtures and form chlorine gas and sodium hydroxide. Themembranes selectively transport the sodium ions across the membrane,while rejecting the chloride ions. ICMs are also useful in the area ofdiffusion dialysis where, for example, caustic solutions are stripped oftheir impurities. The membranes are also useful for their operation invapor permeation and separations due to their ability to transfer polarspecies at a faster rate than non-polar species.

The microporous films must have sufficient strength to be useful inthese various applications. Often this need for increased strengthrequires increased membrane thickness, which can impair the utility ofthe membrane by, for example, decreasing the ionic conductance of ionconductive membranes. Membranes that are inherently weak at smallthicknesses (for example less than 0.050 mm) must be reinforced withadditional materials causing the final product to have increasedthickness.

SUMMARY OF THE INVENTION

The present invention is directed to a shaped microporous article ofpolyvinylidene fluoride, which additionally includes a nucleating agent.The shaped microporous article is oriented in at least one direction ata stretch ratio of at least approximately 1.1 to 1.0. The shaped articlemay also comprise a compound miscible with polyvinylidene fluoride andin which the polyvinylidene fluoride will dissolve at or above themelting temperature of the polyvinylidene fluoride and will phaseseparate upon cooling to a temperature at or below the crystallizationor phase separation temperature of the polyvinylidene fluoride.

The method of making a microporous article comprises the steps of meltblending polyvinylidene fluoride, nucleating agent and glyceryltriacetate; forming a shaped article of the mixture; cooling the shapedarticle to cause crystallization of the polyvinylidene fluoride andphase separation of the polyvinylidene fluoride and glyceryl triacetate;and stretching the shaped article in at least one direction at a stretchratio of at least approximately 1.1 to 1.0.

The present invention is also directed to an ion conductive membranewherein a shaped article of polyvinylidene fluoride and nucleating agentis oriented in at least one direction at a stretch ratio of at leastapproximately 1.1 to 1.0 to provide a network of micropores. The shapedarticle is oriented such that the bubble point pore size is greater thanapproximately 0.4 microns and the shaped article has a thickness lessthan approximately 1.5 mils (37.5 microns) and a Gurley less thanapproximately 10 sec/50 cc. The micropores of the shaped article arefilled with a sufficient quantity of ion conducting electrolyte to allowthe membrane to function as a ion conductive membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an apparatus that may be used topractice the method of and to produce microporous films in accordancewith the present invention.

FIG. 2 is a schematic cross-section of a five layer MEA.

FIG. 3 is a micrograph showing the node and fibril nature of themembrane structure.

FIGS. 4A and B are micrographs of membrane cross-sections showing themicrostructure achieved without and with nucleation. The size of thespherulites varies with larger spherulites being obtained with nonucleation (A) and smaller spherulites with nucleation (B).

FIGS. 5A, B, and C are micrographs showing, respectively, the air side,wheel side, and cross section (wheel side up) of a microporous PVDF filmhaving an asymmetric structure (see Example 9 below).

DETAILED DESCRIPTION

The present invention provides microporous polyvinylidene fluoride(PVDF) films suitable for a variety of applications. The presentinvention applies the process of Thermally Induced Phase Separation(TIPS) to PVDF with selection of a proper diluent and nucleating agentfor the production of microporous films. Glyceryl triacetate issuccessfully used as diluent for production of microporous films fromPVDF. Glyceryl triacetate is readily removed from the microporous filmsby water and is economically and environmentally advantageous because ofthe sewerable, nonhazardous by-products. The invention additionallyprovides several nucleating agents for use in the novel PVDF microporousfilms. Microporous membranes produced from polyvinylidene fluoride(PVDF) using thermally induced phase separation (TIPS) processes can becustomized to have a range of microporous properties, including improvedstrength, chemical resistance, and reduced thickness in comparison toother conventional membrane materials.

A microporous film for a particular application is made by selecting: asuitable thermoplastic polymer; then matching diluent and nucleatingagent to the polymer to achieve the desired properties. If the resultingcast film is of sufficient strength, it is oriented to create thedesired microporous characteristics in the film.

Polyvinylidene fluoride (PVDF) is inherently chemical, UV and fireresistant, low protein binding, and electrically insulating. Therefore,application of this thermoplastic polymer to the development ofmicroporous films is desirable. However, much of the previousdevelopment of microporous films has focused on other thermoplasticssuch as polypropylene. Generally, the diluents and nucleating agents forone class of polymer do not readily extend to other classes of polymers.

A process known as Thermally Induced Phase Separation, or TIPS, is usedto produce the microporous PVDF films of the present invention. Themethod generally involves melt blending a thermoplastic polymer orpolymer blend with a miscible compound, i.e. a diluent, where thediluent is miscible with the thermoplastic at the melting temperature ofthe thermoplastic, but phase separates on cooling below the phaseseparation temperature of the thermoplastic. As used herein, the term“diluent” is meant to encompass both solid and liquid agents. The phaseseparation between the PVDF and diluent may be either liquid-liquid orliquid-solid. After the film or article is phase separated, it isoriented in least one direction to provide a network of interconnectedmicropores throughout. Additionally, the miscible compound (i.e.diluent) may be removed either before or after orientating oralternatively, retained in the film to facilitate filling of the porestructure.

Generally, the TIPS process involves a polymer and a diluent which forma single homogenous phase at an elevated temperature. To process a TIPSfilm, the diluent and polymer are fed into an extruder which heats andmixes the two together to form the homogenous liquid solution. Thissolution is then either cooled in air or, preferably, cast into afilm-like article and cooled upon contact with a casting wheel. Duringthe cooling process for the solid/liquid TIPS constructions, the polymercrystallizes out of the solution to cause the formation of a solidpolymer phase and liquid diluent phase. The solid phase consists ofspherulites held together by polymer chain tie fibrils. In the case of aliquid-liquid TIPS process, the polymer separates out of the solution toform a second liquid phase of polymer-lean material.

After phase separation, the film-like article is typically transparentand can be processed as either a diluent-out or a diluent-in productinto microporous film articles. Diluent-out film is made by extractingsubstantially all of the diluent from the film using a volatile solvent.This solvent is then evaporated away leaving behind air voids where thediluent had been, thus creating a porous film. To increase the air voidvolume, the film is then oriented or stretched in at least one directionand preferably in both the down-web (also called the longitudinal or themachine) and transverse (also called the cross-web) directions.Diluent-in films are made by simply by passing the extraction step andorienting the film. After orienting, the diluent is trapped in theamorphous portions of the polymer and the internal surfaces of theporous structure which makes the porous film dry to the touch. Thismethod also avoids the expensive and rate-limiting extraction step.

In particular, the TIPS process involves four steps:

(1) melt blending to form a solution comprising about 10 to 90 parts byweight of a polymer component, and about 90 to 10 parts by weight, basedon a total solution content, of a diluent component, said diluentcomponent being miscible with the polymer component at a temperatureabove the melting temperature of the polymer component, or theliquid-liquid phase separation temperature of the total solution;

(2) shaping the solution;

(3) phase separating the shaped solution to form phase separatedmaterial, i.e., polymer, regions through either (i) crystallization ofthe polymer component to form a network of polymer domains, or (ii)liquid-liquid phase separation to form networks of a polymer-lean phase;and

(4) creating regions of air adjacent to the material regions to form theporous article.

The structure can be varied by manipulation of six process variables:(1) quench rate (time for the polymer/diluent solution to cool and phaseseparate), (2) heterogeneous nucleating agent presence and concentration(useful with solid/liquid TIPS), (3) polymer component to diluentcomponent weight ratio, (4) stretch, (5) diluent extraction, and (6)annealing.

The phase separation step to form the desirable size of material regionscan be carried out by (1) cooling the solution fast enough, (2) usingnucleating agents (with solid/liquid TIPS), or (3) a combination ofboth. In TIPS, cooling can be achieved by maximizing the intimatecontact of the hot solution to a quenching surface or medium. Typicallymicroporous films made by the solid/liquid TIPS process are cooled bycasting on a patterned roll. Alternatively, microporous TIPS may also becast on a smooth wheel; the desired performance properties woulddetermine the preferred quenching method. (The TIPS process is alsodescribed in, for example, U.S. Pat. No. 5,976,686, which isincorporated herein by reference.)

The PVDF utilized in the articles and process of the present inventionis not limited to a single PVDF polymer. References to PVDF generallyinclude PVDF resin, homopolymers, co-polymers, and polymer blends wherethe majority polymer is PVDF. PVDF also includes or refers to closelyrelated derivatives of PVDF. Examples of PVDF resins suitable for use inthe present invention are available from Solvay Solexis of Thorofare,N.J. under the tradenames HYLAR and SOLEF and from AtoFina Chemicals,Inc. of Philadelphia, Pa. under the tradename KYNAR. Individual resinsare listed in the examples section below. These PVDF resins generallyhave crystallinity ratios of approximately 0.3 to approximately 0.4, butthe invention is not so limited. Additionally, the PVDF resin employedmay vary in properties such as molecular weight and melt flow. The meltflow indices generally varied between approximately 0.13 toapproximately 6.0 at 230° C., 5 kg. Although it is known that longerchains or lower melt flow may increase the strength of the resultingarticle, the invention is not so limited.

The microporous PVDF films are produced from the TIPS process utilizingglyceryl triacetate as the diluent. Glyceryl triacetate, in addition tofunctioning as a diluent for PVDF in TIPS processes, also has additionaladvantages related to its non-hazardous nature. Glyceryl triacetate hasbeen previously used as a food additive and is therefore non-toxic. Theglyceryl triacetate can remain in the film or be removed eitherpartially or almost completely. Glyceryl triacetate can readily beremoved from PVDF microporous membranes using water as a solvent.Additionally, the by-products or effluents are glycerol and acetic acid,both of which are also non-toxic and sewerable. There are considerableeconomic and environmental advantages to not requiring or producingorganic solvents that must be disposed of during the removal process.

A preferred range of PVDF to diluent in the present invention rangesfrom approximately 60:40 to approximately 40:60 depending on theproperties desired. Glyceryl triacetate used as a diluent in the presentinvention is available from Eastman Kodak Company of Rochester, N.Y.under the tradename TRIACETIN. A particularly preferred range of PVDF toglyceryl triacetate is approximately 50:50 to 40:60.

The method of the present invention by the TIPS process also usesnucleating agents to manipulate and improve the properties of PVDFfilms. Nucleating agents generally enhance the initiation ofcrystallization sites and induce crystallization of the PVDF from theliquid state thereby increasing the rate of crystallization. Theincrease in rate of crystallization generally causes a reduction in thesize of the spherulites or particles of the crystallized polymer. Thus,the nucleating agent employed should be a solid at the crystallizationtemperature of the polymer. Evidence of successful size reduction of thespherulites does not by itself guarantee that the nucleating agent hasfacilitated the production of a PVDF film that has sufficient strengthto withstand the orienting to develop the microporous structure.

The use of nucleating agent in accordance with the present inventionsubstantially accelerates the crystallization of the PVDF, therebyresulting in a more uniform, stronger microstructure. The stronger, moreuniform microstructure from successful nucleation of the PVDF has anincreased number of tie fibrils per unit volume and allows for greaterstretching or orienting of the film so as to provide higher porosity andgreater tensile strength than previously achievable. These propertiesadditionally facilitate the use of thinner membranes, less than 2.0 mil(50 μm), with the membranes being sufficiently strong so as to notrequire reinforcement. Although nucleating agents for use in TIPSprocesses are known in conjunction with the production of microporousarticles from other polymer types, those nucleating agents did notreadily extend to the production of microporous membranes from PVDF.

The present invention utilizes particular members of a class of pigmentscalled “vat type” pigments to nucleate the PVDF. These organic pigmentsinclude anthraquinones, perylenes, flavanthrones, and indanthrones. TheColour Index (CI) identifies each pigment by giving the compound aunique “Colour Index Name (CI Name) and a “Colour Index Number” (CINumber). Classification of pigments can be done by grouping pigments byeither chemical constitution or by coloristic properties. Some pigmentsare “nonclassifed”, for example Indanthrone Blue (CI 69800 Pigment Blue60). Perylene pigments include the dianhydride and diimide of perylenetetracarboxylic acid along with derivative of the diimide. Anthraquinonepigments are structurally or synthetically derived from theanthraquinone molecule.

Materials found to successfully nucleate PVDF include, but are notlimited to: CI 67300 Vat Yellow 2, designated as Indanthrene Yellow GCNavailable from TCI America of Portland, Oreg. (“TCI”); CI 70600 PigmentYellow 24, designated as Flavanthrone also available from TCI; CI 69800Pigment Blue 60, an indanthrone available from Ciba Specialty Chemicalsof Hawthorne, N.Y. under the trademark CHROMOPHTAL Blue A3R; CI 71130Pigment Red 179, a perylene available from Bayer Corporation—Coatingsand Colorants of Pittsburgh, Pa. under the trademark PERRINDO MaroonR-6438; and CI 58055:1 Pigment Violet 5:1, an anthraquinone availablefrom Bayer Corporation—Coatings and Colorants of Pittsburgh, Pa. underthe trademark FANCHON Maroon MV7013. The preferred vat type pigmentsused as nucleating agents are Pigment Blue 60 (CI No. 69800), PigmentRed 179 (CI No. 71130), Pigment Violet 5:1 (CI No. 58055:1), Vat Yellow2 (CI No 67300), and Pigment Yellow 24 (CI No. 70600).

Additionally, it was determined that nanometer-sized particles ofpolytetrafluoroethylene (PTFE) also successfully nucleate PVDF. PTFE hadpreviously been used to produce dense, non-porous PVDF films in contrastto the microporous films of the present invention. To successfullynucleate PVDF for the production of microporous films, the nanometersized particles of PTFE need to be dispersed evenly throughout the PVDF.Therefore, aggregation of the PTFE particles is not desirable. Examplesof suitable methods for dispersing PTFE for use as a nucleating agent inthe present invention are described below.

One method is to suspend nanometer-sized particles of PTFE in an aqueoussolution such as DYNEON TF-5235 available from Dyneon Corp. of Oakdale,Minn. The dispersion of PTFE nanoparicles is coated/spread onto the PVDFresin pellets. The PVDF pellets are then dried, leaving the PTFE coatedon the resin pellets prior to their use in the TIPS process. Thesolution is not limited to water. Any solution that does not otherwisereact with the resin pellets or PTFE and that may be volatilized orremain in the resulting article without effect may be used. The separatedrying step may be omitted where proper venting is incorporated duringmelting of the resin during the TIPS process.

Another method uses nanometer-sized particles of PTFE in the form ofMetablen A-3000 available from Mitsubishi Rayon Corporation of New York,N.Y. The PTFE particles are wrapped in a second polymer wherein thesecond polymer is miscible in the melt blend (described below). Oneexample is nanometer-sized PTFE particles wrapped inpolymethylmethacrylate (PMMA). The PMMA aids in dispersion of the PTFEparticles as the PVDF resin, diluent and PTFE (in the form of MetablenA-3000) is mixed. The PTFE sheds the PMMA coat as the PMMA melts duringthe TIPS process. The PTFE particles do not melt and are exposed tonucleate PVDF crystallization.

Generally, a nucleating agent is premixed with the diluent, oralternatively, the resin prior to melt blending the mixture during theTIPS process. For example, the pigments used as nucleating agents in thepresent invention may be mixed with the glyceryl triacetate on aroto-shear mixer or, a Mini-Zeta bead mill. The PTFE is pre-compoundedas described above and fed via a pellet feeder, or alternatively, fedinto the melt blend via a powder feeder.

Additionally, certain conventional additives may be blended with thePVDF, or glyceryl triacetate and/or melt blend thereof. When used, theconventional additives should be limited in quantity so as not tointerfere with the formation of the microporous films and so not toresult in unwanted exuding of the additive. Such additives may includeanti-static materials, dyes, plasticizers, UV absorbers, and the like.The amount of additives is typically less than 10 percent of the weightof the polymer components, preferably less than 2 percent by weight.

The use of PVDF, glyceryl triacetate, and specific nucleating agents tomanufacture the microporous PVDF films are further described below inthe context of the TIPS process.

First, a melt blend comprising a mixture of PVDF resin, glyceryltriacetate and nucleating agent is prepared. Various components may bepre-compounded prior to melting, for example, the nucleating agentsdescribed herein. The term of melt blend refers to the PVDF polymer,glyceryl triacetate and nucleating agent blend, wherein at least thePVDF and glyceryl triacetate are in the molten, semi-liquid or liquidstate. The melt blend is prepared by mixing approximately 40 to 60percent by weight of crystallizable PVDF thermoplastic polymer withapproximately 60 to 40 percent by weight of glyceryl triacetate, andadditionally including a nucleating agent. The nucleating agentrepresents about 0.1 to about 1%, more particularly about 0.25 to about1% by total weight of the melt blend. Alternatively, the nucleatingagent represents about 0.2 to about 2.5% by weight of the polymer. Themelt blend is heated to at least the melting temperature of the PVDF.For ease of handling the melt blend and ease in its casting, it isconvenient to initiate the formation of the melt blend by heating themixture at a temperature in a range of about 25° C. to about 100° C.above the melting temperature of the PVDF.

Microporous films of the present invention are prepared by casting theshaped article, such as a sheet or layer, from the melt blend comprisingthe PVDF, glyceryl triacetate and nucleating agent. The properties ofthe microporous films are dependent upon, but not limited to, the ratioof polymer to diluent in the melt blend, the type and amount ofnucleating agent employed, the rate of cooling, and the stretch ratiosand temperatures. During cooling, heat is removed from the melt blenduntil the crystallization temperature of the PVDF in the melt blend isreached, and controlled crystallization and phase separation of the PVDFcan begin. Cooling temperatures greater than about 125° C. below theequilibrium melting point of the pure crystallizable PVDF polymer causetoo rapid quenching of the melt blend. The materials can be rendereduniformly microporous by orienting, but are inherently weak in thewashed or diluent-out condition compared to those quenched at highertemperatures. By contrast, U.S. Pat. No. 4,539,256 describes coolingtemperatures more than about 225° C. below the equilibrium melting pointof the pure crystallizable PVDF polymer cause too rapid quenching of themelt blend and could result in single-phase films, which although strongand transparent, are substantially incapable of being rendered uniformlymicroporous by orienting. At the desired high diluent levels, castingwheel temperatures of less than about 74° C. below the equilibriummelting point of the pure crystallizable PVDF polymer provides for tooslow a phase separation (crystallization) of the PVDF, which, withoutadditional quenching lubricants such as TRIACETIN or water, will causethe material to adhere to the patterned wheel. Thus, coolingtemperatures between approximately 71° C. to 131° C. below theequilibrium melting point of the pure crystallizable PVDF are obtainablewithout process modifications with the preferred temperatures between82° C. and 124° C. below the equilibrium melting point of the purecrystallizable PVDF.

One method is to cool the cast article in a quench bath at anappropriate temperature. Another method is to use a casting wheel wherethe temperature of the wheel is controlled to within the desired coolingtemperature range similar to the quench bath.

The cast films formed from the TIPS process are generally solid andtransparent before the optional diluent removal and orienting. Themicrostructure of the cast films may be described as spherulites andaggregates of spherulites of the PVDF with glyceryl triacetate occupyingthe space between particles (See FIG. 3 and FIGS. 4A and B). Adjacentspherulites and aggregates of PVDF are distinct, but they have aplurality of zones of continuity. The PVDF spherulites and aggregatesare generally surrounded or coated by the glyceryl triacetate, but notcompletely. There are areas of contact between adjacent PVDF spherulitesand aggregates where there is a continuum of PVDF from onespherulite/aggregate to the next adjacent spherulite/aggregate in suchzones of continuity.

On orienting, the PVDF spherulites and aggregates are pulled apart,permanently attenuating the polymer in zones of continuity; therebyforming the fibrils, forming minute voids between coated spherulites andaggregates, and creating a network of interconnected micropores. As usedherein, “orienting” means such stretching beyond the elastic limit so asto introduce permanent set or elongation of the article, typically toobtain at least an increase in length of about 10% or expressed as aratio, approximately 1.1 to 1.0. Stretching to provide an elongation ofabout 10% to about 1000% is typical. The actual amount of stretchingrequired will depend upon the composition of the film and the degree ofporosity (for example pore size) desired.

Stretching may be provided by any suitable device that can providestretching in at least one direction and may provide stretching both inthat direction and in the transverse direction. The stretching may bedone sequentially or simultaneously in both directions. For example, afilm may be oriented in both the machine direction and the transversedirection. Stretching should be uniform to obtain uniform and controlledporosity. Stretching in one direction typically causes the film tonarrow or “neck” in the cross direction so stretching a film to providean elongation of 50%, for example, does not produce a 50% increase inthe surface area of the film.

Such permanent attenuation also renders the article permanentlytranslucent. Also on orienting, if the diluent is not removed, thediluent remains coated on or surrounds, at least partially, the surfacesof the resultant PVDF particles. Typically, the microporous films aredimensionally stabilized according to conventional well-known techniquesby heating the oriented film while it is restrained at aheat-stabilizing temperature. This is also referred to annealing.

The nucleated films have a microporous structure characterized by amultiplicity of spaced (that is separated from one another), randomlydispersed, non-uniform shaped, equiaxed particles of PVDF connected byfibrils, with nucleating agent in the interior of the particles.(Equiaxed means having approximately equal dimensions in alldirections.) If the diluent is not removed, the particle of PVDF is alsocoated with the glyceryl triacetate.

Where the glyceryl triacetate diluent is removed from the microporousfilm, a unique microporous sheet of PVDF with nucleating agentincorporated therein remains. The resultant microporous film may beimbibed with various materials to provide any one of a variety ofspecific functions thereby providing unique articles. The film may beimbibed after removing the glyceryl triacetate, or alternatively, theglyceryl triacetate may be left in the microporous PVDF film prior tothe imbibing process. Several methods are known for imbibing microporousfilms including: multiple dipping, long soak, vacuum, hydraulic pressand evaporation. Examples of imbibing materials that might be employedin the present invention include, but are not limited to:pharmaceuticals, fragrances, antistatic agents, surfactants, pesticides,and solid particulate materials. Certain materials, such as antistaticagents or surfactants, may be imbibed without prior removal of theglyceryl triacetate diluent.

The microporous film, either before or after removal of the diluent, maybe further modified by depositing any one of a variety of compositionsthereon using any one of a variety of known coating or depositiontechniques. For example, the microporous film may be coated with metalby vapor deposition or sputtering techniques, or coated with adhesives,aqueous or solvent base coating compositions or dyes. Coating may beaccomplished by conventional techniques such as roll coating, spraycoating, dip coating or any other coating techniques.

The microporous films may be laminated to any one of a variety of otherstructures, such as other sheet materials to provide compositestructures. Lamination can be accomplished by conventional techniquessuch as adhesive bonding, spot welding, or by other techniques that donot destroy or otherwise interfere with the porosity or createundesirable porosity or perforations.

The microporous PVDF of the present invention is generally in the formof a sheet or film, although other article shapes are contemplated andmay be formed. For example, the article may be in the form of a sheet,tube, filament, or hollow fiber.

Polyvinylidene fluoride (PVDF) microporous membranes are described belowfor use as ion conductive membranes (ICMs), including ion exchangemembranes. However, PVDF membranes are not restricted to that use. Theirchemical stability and relative strength are also useful in variousfiltering applications. The microporous films made through thisinvention may be used in a variety of applications, such as, ionconductive membranes, separators in electrochemical cells, diffusionbarriers, viral barriers, absorbent clothes, and ultrafiltration ofcolloidal matter. In addition, PVDF membranes have low specific proteinbinding that may be useful in biotechnology related applications. PVDFmembranes are also inherently flame retardant without the addition ofother chemicals, which can be a cost saving device in applicationsrequiring that characteristic.

The properties of PVDF microporous films when customized for use as ionconductive membranes offer advantages over prior art ion conductivemembranes. The prior art single component membranes have two primaryproblems: strength and stability. The membranes themselves are oftenvery fragile. Therefore, such membranes generally must have increasedthickness and/or must either be mounted or otherwise attached to asupport to avoid puncture and/or tearing. Additionally, conventionalmembranes made solely of polymer electrolytes are very expensive andextremely fragile.

Ion conductive membranes form a gaseous barrier blocking flow of thereactants within an electrochemical cell structure, while providingionic conductivity between an anode and a cathode located on oppositesides of the membrane. Ion conductive membranes may be conductive ofonly ions either of positive charge or negative charge, that is, eithera cation exchange membrane or an anion exchange membrane; or only of onetype of ion, for example, a proton exchange membrane. Proton exchangemembranes (PEMs) are one type of ion conductive membranes used inmembrane electrode assemblies that can be used to produce fuel cells,electrolyzers and electrochemical reactors. The present disclosurefocuses on composite ion conductive membranes, including proton exchangemembranes, for use in fuel cells, although the analogy to electrolyzersand electrochemical reactors is straightforward.

An example five layer MEA generally indicated as 50 is illustrated inFIG. 2. The various layers for the electrochemical oxidation of a fueland reduction of an oxidizing agent to produce electric current include:an ion conductive membrane 52, catalyst layers 54, 56, and electrodebacking layers 58, 60. The shape and size of the components of theelectrochemical cell can vary over a wide range depending on theparticular design. MEAs can include: porous metal films or planardistributions of metal particles or carbon supported catalyst powdersdeposited on the surface The typical flow of reactants is additionallydepicted in FIG. 2.

A composite ion conductive membrane is made by imbibing the microporousstructure of the microporous PVDF film with an ion conductive material.The composite ICMs offer superior properties over single componentmembranes when used in MEAs. The composite ICMs can be made thinner andstronger while giving equivalent conductivity with less electrolyte, andhave more dimensional stability even after becoming saturated withwater. However, because the membranes employed are initially porous, thegas permeability of the resulting membrane depends in part on the degreeto which the membrane is filled by the electrolyte.

While being conductive of protons or other ions, the ion conductivemembrane comprising a microporous PVDF film is nonconductive withrespect to electrons and gaseous reactants. To prevent the passage ofgaseous reactants, the ion conductive membrane should have sufficientthickness for mechanical stability and should be effectivelynonpermeable (pinhole free). Conduction of gaseous reactants through theion conductive membrane could result in undesirable direct reaction ofthe reactants. Similarly, the conduction of electrons through protonexchange membrane could result in an undesirable short circuit of thecell. PVDF is advantageously non-conductive. If the membrane failscausing direct reaction of the reactants or a short circuit, the energyreleased by the reaction of the fuel with the oxidizing agent cannot beused to produce electricity, thereby defeating the purpose of themembrane electrode assembly.

Pore size of the microporous films is controlled in ICM applications.The effective pore size is at least several times the mean free path ofthe flowing molecules and may be varied with the range of approximatelyfrom several micrometers down to about 100 Å. In ICMs, the pore sizeneeds to be large enough that the electrolyte is able to migrate intothe membrane. For example, a pore size greater than approximately 0.4microns is suitable. It is desirable that the electrolyte fills ornearly completely fills the pores of the microporous membranes. If thepore size of the membrane is too small, the membrane will actually actas a filter during the electrolyte introduction process, therebyresulting in a nonfunctional or a poorly functional ICM. The upper endof the pore size range will also be controlled because of issues ofmembrane strength and electrolyte retention within the membrane.

In one implementation of the invention, the PVDF microporous films aresuitably impregnated with the ion conducting electrolyte, effectivelyfilling the interior volume of the membrane for use as a PEM or othertype of ion conductive membrane. The ion conducting electrolyte shouldbe chemically stable and compatible with the catalysts used in the MEAso that the catalyst is not poisoned. The ion conducting electrolyte ispreferably a polymer electrolyte, frequently referred to as an ionomer.Polymer electrolytes can be made from a variety of polymers including,for example, polyethylene oxide, poly (ethylene succinate), poly(β-propiolactone), and sulfonated fluoropolymers such as NAFIONcommercially available from E.I. DuPont De Nemours and Company,Wilmington, Del.

The amount of electrolyte solution used in filling the microporous filmshould be sufficient to achieve the degree of filling desired but ispreferably in excess of that which would theoretically fill themembrane. The amount of electrolyte imbibed in the pores or adsorbed onthe fibrils of the membrane after the filling should be sufficient tofill between 95% and 100% of the available pore volume. Preferably, morethan 95% of the available pore volume is filled. Most preferably,between 95% and 100% of the available pore volume is filled. Theelectrolyte may be present as a coating on the structural fibrils of theporous membrane or it may wet out the membrane filling the entirecross-section of some pores.

The electrolyte solution used for imbibing the microporous membrane ismore accurately described as a dispersion of the ion conductingelectrolyte with particles of approximately 260 Å, as determined bymeasurements of radius of gyration of the micelles using Small AngleX-ray scattering (SAXS), in solution at a concentration of typically 5to 20 weight percent. It is important that the pores of the microporousmembranes be of adequate size to allow the electrolyte to enter themembrane. If the pore size is too small, the microporous membraneinstead acts as a filter removing the electrolyte from solution and atthe same time failing to incorporate the electrolyte into the pores ofthe membrane. The properties of the electrolyte are considered indetermining the required pore size. Ion conducting electrolytes withhigher molecular weights and/or that are heavily branched orcross-linked will typically require larger pore sizes than lowermolecular weight, linear molecules.

It is not necessary to remove the glyceryl triacetate diluent beforeimbibing prior to imbibing the PVDF membrane with ion conductingelectrolyte. The microporous PVDF membranes in which the diluent has notbeen removed are referred to as “diluent in”. Microporous PVDF membraneswith the diluent removed, referred to as “diluent out” may also besuccessfully imbibed with ion conducting electrolyte for use as a PEM.

The microporous PVDF membranes can become hydrophobic after removal ofthe diluent. To assist filling of a hydrophobic, diluent out membranewith the electrolyte solution, which is commonly aqueous and/or ionic innature, the membrane is treated prior to filling. Techniques that may beused include: pre-wetting, radiation grafting, corona treating, or otherchemical treatment. For example, the diluent out membranes may bepre-wet with a solution of n-propanol and glycerol. The excessn-propanol/glycerol can be removed by squeegee prior to laying themicroporous membrane into the dispensed electrolyte solution.

EXAMPLES

The following examples are given to show microporous materials that havebeen made in accordance with the present invention. However, it will beunderstood that the following examples are exemplary only, and are notintended to be comprehensive of the many different microporous materialswhich may be made in accordance with the present invention

Materials

PVDF Polymers:

HYLAR MP-20 1,1-difluoroethane based polymer, 166-170° C. MeltTemperature, 1.57 Melt Flow Index (Solvay Solexis, Thorofare, N.J.)

HYLAR MP-32 1,1-difluoroethane based polymer, 166-170° C. MeltTemperature, 0.13 Melt Flow Index (Solvay Solexis, Thorofare, N.J.)

Kynar 1000HD 1,1-difluoroethene based polymer, 166-170° C. MeltTemperature, 1.86 Melt Flow Index (Atofina Chemicals Philadelphia, Pa.)

SOLEF 1010 PVDF polymer, 170-175° C. Melt Temperature, 5.33 Melt FlowIndex (Solvay Solexis, Thorofare, N.J.)

SOLEF 1012 PVDF polymer, 170-175° C. Melt Temperature, 1.3 Melt FlowIndex (Solvay Solexis, Thorofare, N.J.)

SOLEF 1015 PVDF polymer, 170-175° C. Melt Temperature, 0.14 Melt FlowIndex (Solvay Solexis, Thorofare, N.J.)

Diluents:

TRIACETIN glyceryl triacetate (Eastman Kodak Co., Rochester, N.Y.)

Nucleating Agents:

CI 69800, Pigment Blue 60, Indanthrone, CHROMOPHTAL Blue A3R (CibaSpecialty Chemicals, Hawthorne, N.Y.)

CI 71130, Pigment Red 179, Perylene, PERRINDO Maroon R-6438 (BayerCoatings and Colorants Corp., Pittsburgh, Pa.)

METABLEN A-3000 nanometer sized PTFE particles coated withpolymethylmethacrylate (Mitsubishi Rayon Corp., New York, N.Y.)

DYNEON TF-5235: An aqueous dispersion of 225 nm PTFE particles. (DyneonCorp., Oakdale, Minn.)

Ion Conducting Electrolytes:

NAFION 1000: A solution of a hydrolyzed copolymer ofpolytetrafluoroethylene with perfluorosulfonylethoxyvinylether withconversion of its sulfonyl radical to a sulfonic radical. Solutioncomposition: 21.53% solids, 21.33% water, 22.20% ethanol, 33.71%propanol and 1.23% other. (DuPont Chemicals Company, Wilmington, Del.)

Test Methods

Tensile Strength:

The tensile strength at break was measured according to ASTM D882 usingan INSTRON Model 1122 tensile tester with a crosshead speed of 25 cm/minand a gage length of 5 cm. The width of the test specimens was 2.5 cm.The tensile strength at the break point of the sample was calculated bydividing the load (force) at the break point by the originalcross-sectional area of the specimen and is reported in kg-force/cm².The percent elongation at the break point was calculated by dividing theelongation at the break point by the original gage length andmultiplying by 100.

Gurley Porosity:

Gurley is a measure of the resistance to gas flow of a membrane,expressed as the time necessary for a given volume of gas to passthrough a standard area of the membrane under standard conditions, asspecified in ASTM D726-58, Method A. Gurley is the time in seconds for50 cubic centimeters (cc) of air, or another specified volume, to passthrough 6.35 cm² (one square inch) of the membrane at a pressure of 124mm of water. The film sample was clamped between cylindrical rings, theuppermost of which contained a piston and the specified volume of air,when released, the piston applied pressure, under its own weight, to theair in the upper cylinder and the time taken for the specified volume ofair to pass through the membrane was measured. A membrane with a Gurleyvalue less than approximately 10 sec/50 cc is preferred for producingcomposite ion conductive membranes.

Bubble Point:

The Bubble Point pore size is the bubble point value representing thelargest effective pore size measured in microns according toASTM-F-316-80.

H Pump:

The H pump test applies a current causing the hydrogen fuel to splitinto protons on the anode side of the membrane electrode assembly. Theprotons pass through the membrane and recombine on the cathode side tomake hydrogen—H₂. This hydrogen to hydrogen reaction to split andrecombine hydrogen is used for diagnostics. H pump measures theresistance of the hydrogen ions as they move through the membrane in thez-axis (normal to the membrane). It is possible to use the H pump valueto calculate the membrane resistance.

H₂ Crossover:

H₂ crossover measures the diffusion of hydrogen through the membrane.Higher values of H₂ crossover generally indicate greater diffusion ofH₂. The composite PVDF ICMs typically have less diffusion of hydrogencompared to the conventional dense ionomer membranes. H₂ crossover isalso used to determine if there are any pinholes in the membrane priorto assembly into an MEA.

MEA Performance 0.8V & 0.6V:

The values for Performance at 0.8V and Performance at 0.6V are used toevaluate the performance of an MEA by obtaining a voltage versus currentplot or polarization curve of the MEA at 70° C. These cells were runwith 100% RH (relative humidity) hydrogen and 100% RH air. The catalystloading was 0.4 mg/cm² on both the anode and cathode. The resultantpolarization curve related the current flowing through the cell to thepotential difference across the cell.

Puncture Strength:

The Puncture Strength Test measures the puncture peak load of amembrane. In particular, the Puncture Strength Test was used to measurethe puncture peak load of a cast NAFION membrane or a compositePVDF/NAFION membrane. The instrument used was an INSTRON Series 5500Rwith a ION load cell. The puncture tip is a NAJET Mandrel Mounted Series#400—Material HSSC (Item 0201) shank measuring 0.040″ (0.10 cm) mountedon a 0.090″ (0.23 cm) mandrel with a tip diameter of 0.0030″ (0.0076cm). The puncture strength measures the force per square centimeternecessary to puncture a film of known thickness. The sample size was1.5″ (3.8 cm)×1.5″ (3.8 cm) square. The crosshead speed was 2 mm/min.

Example 1-7

Microporous PVDF films were prepared using apparatus similar to thatshown in FIG. 1 using the method described below. The properties ofthese films are shown below in TABLE 1. TABLE 1 illustrates theeffectiveness of CI 69800 Pigment Blue 60, CI 71130 Pigment Red 179 andPTFE as nucleating agents compared to a non-nucleated control PVDF film.The nucleating agent used and the amount were varied between the filmsand is noted in TABLE 1 as percent by total weight of the melt blend.The thickness of each film was measured and is shown in TABLE 1. Thebreak tensile strength of the films in both the machine direction (MD)and transverse direction (TD) was measured. The nucleated filmsdemonstrated superior break strength and elongation compared to thenon-nucleated control which could not be oriented to any appreciableextent.

With reference to FIG. 1, the PVDF polymer pellets (SOLEF 1012) wereintroduced into the hopper 12 of a 25 mm co-rotating twin-screw extruderwith an approximate total extrusion rate of 3.6-4.5 kilograms per hourand a screw speed of 150 RPM. For Examples 1-5, the nucleating agent, inpowder form, was premixed with the glyceryl triacetate diluent in aMini-Zeta bead mill and then fed, with additional diluent by a feedingdevice 13 into the extruder 10 via a port 11 in the extruder wallintermediate the hopper 12 and the extruder exit 30. For Examples 6-7,the nucleating agent, in dispersion form, was coated onto the PVDFpellets as previously described and then fed via the hopper 12. Thepolymer to diluent ratio was varied slightly in accordance with theamount of nucleator used, but was generally approximately 0.41:1.0. Theextruder had eight zones with a temperature profile of zone 1 at 204°C., zone 2 at 266° C., zone 3 at 266° C., zone 4 at 221° C., zone 5 at182° C., zone 6 at 182° C., zone 7 at 182° C. and zone 8 at 182° C.(shown in FIG. 1 as zones 16, 18, 20, 22, 24, 26, and 28, respectively).The melt was subsequently pumped through a double-chromed coat-hangerslot film die 32, cast onto a chrome roll 36 that ranged from 52° C. forExample 2 to 63° C. for Examples 1 and 3-7, and then wound into a roll.Film samples were cut from the rolls and placed in metal framesmeasuring 15 cm by 28 cm. The frames were then placed in small pans ofdeionized water for 20 minutes (effectively removing the TRIACETINdiluent from the films) and then allowed to dry in ambient air. Thewashed film samples were then stretched biaxially 1.75 by 1.75 on a TMLong Film Stretcher (TM Long Co., Somerville, N.J.) at 132° C. The filmswere held in the stretcher for 2-5 minutes at 132° C. after stretchingwas complete to anneal the film.

Alternatively, the layer 34 could be fed into a liquid quench bath or agas quench bath, maintained at a suitable temperature below thecrystallization temperature of PVDF using water or other suitablesolvent. Where the quench bath 38 uses water gas or other suitablesolvent, the bath functions to remove the glyceryl triacetate diluent.The film is then directed to a machine-direction stretching device 42and a transverse-direction stretching device 44, and then to a take-uproller 46 for winding into a roll. TABLE 1 MD MD TD TD Break Break BreakBreak % Nucleating Thickness stress Elong stress Elong Example Agent(μm) kg/cm² % kg/cm² % Control 1 None — — — — — 1 0.25 P.B. 60 54.6 10419.5 112 18.1 2 0.5 P.B. 60 57.1 121 34.8 138 29.2 3 0.25 P.R. 179 42.9129 13.7 126 12.8 4 0.5 P.R. 179 49.2 111 12.7 119 12.8 5 1.0 P.R. 17946.7 105 15.2 91 11.7 6 0.25 PTFE 57.5 102 15.7 116 12.9 7 0.5 PTFE 54.6111 17.2 80 9.0

Examples 8-19

Microporous PVDF films of Examples 10-17 and 19 were prepared as inExamples 1-7 (i.e. samples cut and placed in frames measuring 15 cm by28 cm and then places in small pans of deionized water for 20 minutes).The type of PVDF resin, the polymer to diluent ratio, the chrome rolltemperature (i.e. the rate of cooling), and the stretch ratios andtemperatures, were varied to produce a range of properties as shown inTABLE 2 below. P.B. 60 was used as the nucleator at various percentagesby total weight of the melt blend. Examples 8-12 and 14 had a prewashedTRIACETIN content of 50% by weight, Example 13 had a prewashed TRIACETINcontent of 55% by weight, and Examples 15-19 had a prewashed TRIACETINcontent of 58% by weight. The chrome roll was varied from 52° C. to 82°C. Subsequent to the chrome roll, the quenched extrudate of Examples 8,9, and 18 was fed through a water wash bath maintained at ambienttemperature (22° C.) to remove the glyceryl triacetate diluent.

The washed film samples of Examples 10-17 and 19 were then stretchedbiaxially 2×2 on a TM Long Film Stretcher at 132° C. The films were heldin the stretcher for 2-5 minutes at 132° C. after stretching wascomplete to anneal the film. Examples 8-9 were stretched in-linebiaxially 2×2 and Example 18 was stretched biaxially 1.7×1.85 on alength orienter 42 and tenter 44 as shown in FIG. 1. TABLE 2 % P.B. 60Chrome Nucleating Thickness roll Temp Gurley, B.P. Pore Example AgentPolymer (μm) (° C.) sec/50 cc Size, μm  8 0.5 SOLEF 1010 46 82 >>600 — 9* 0.5 SOLEF 1010 61 60 166.0 10 0.5 HYLAR MP-32 41 52 148.9 0.19 110.5 HYLAR MP-32 38 63 155.7 0.26 12 0.5 HYLAR MP-32 36 52 127.2 0.32 130.5 HYLAR MP-32 43 57 30.7 0.58 14 0.25 SOLEF 1012 41 52 96.9 0.84 150.5 SOLEF 1012 46 52 7.0 1.01 16 0.5 SOLEF 1012 48 63 6.0 1.14 17 0.5HYLAR MP-32 46 63 7.0 1.28 18 0.25 SOLEF 1012 23 63 3.5 1.54 19 0.5HYLAR MP-32 43 63 4.5 1.66*Example 9 resulted in an asymmetric structure having a thin skin layerwith relatively small pores and a thick core layer with relatively largepores. The skin layer corresponded to the side of the extrudate castagainst the chrome roll (see FIGS. 5A, B, and C).

Examples 20-25

Microporous PVDF films for use as PEMs were prepared as in Examples10-17 and 19 except the films were produced on a 40 mm co-rotatingtwin-screw extruder using an extrusion rate of 9 to 11.4 kilograms perhour and a barrel temperature profile of 216° C., 271° C., 221° C., 188°C., 188° C., 188° C., and 188° C. A screw speed of 150 RPM was used. Themelt blend comprised PVDF and glyceryl triacetate diluent in a ratioranging from approximately from 45:55 to approximately 40:60. CI 69800Pigment Blue 60 was added to the melt blend in a concentration ofapproximately 0.4 percent by total weight of the melt blend. The meltblend was pumped through a double-chromed coat-hanger slot film die ontoa cooled, patterned chrome roll. The chrome roll was engraved with aseries of intersecting knurling lines (25 lines/cm) resulting in aseries of raised pyramidal structures across the roll face having aheight of approximately 140 microns. The chrome roll temperature rangedfrom approximately 35° C. to approximately 74° C. The films were notwashed to remove the diluent. The films were then oriented as inExamples 8-9 and 18.

The PVDF microporous films were then impregnated with an ion conductingelectrolyte (NAFION 1000), effectively filling the interior volume ofthe membrane for use as a PEM.

The microporous PVDF membranes were imbibed by coating each side of themembrane with equal amounts of NAFION 1000 ion conducting electrolytesolution using the following technique. Two glass plates were cleanedwith isopropanol and water in a 50:50 solution. The microporous PVDFmembrane was taped along an edge of a first plate and then laid over anadjacent second plate. Using an applicator gage, a controlled amount ofthe ion conducting solution was dispensed onto the first plate. Themicroporous membrane was laid back over the first plate; carefully intothe dispensed electrolyte so as to avoid any ripples and bubbles. Theelectrolyte solution was allowed to diffuse through the membrane for10-30 seconds. A controlled amount of electrolyte solution was thendispensed onto the microporous membrane supported by the first plate.The microporous membrane was then dried at 90° C. for 10 minutes andannealed at 160° C. for 10 minutes. The resulting ion conductivemembrane (ICM) was subsequently assembled into a membrane electrodeassembly for testing via the above described test methods. The membraneswere hydrophilic in nature, which facilitated the coating of themembrane with the ionomer solution. Porosity and electrical propertiesof the ICMs are shown in TABLES 3 and 4 below. TABLE 3 Casting % GurleyWheel TRIAC- sec/50 cc Thick- Exam- Temperature ETIN (before B.P. Poreness ple (° C.) diluent filling) Size, μm (μm) 20 74 56 35.2 1.06 18 2163 56 24.3 1.25 18 22 63 58 11.1 1.39 15 23 63 58 11.1 1.39 20 24 74 5635.2 1.06 13 25 63 58 11.1 1.39 31

TABLE 4 H₂ crossover Performance Performance H pump (10/0 psi) at 0.8 Vat 0.6 V Example mOhm-cm² mA/cm² (A/cm²) (A/cm²) 20 300 1.1 0.12 0.43 21270 2.5 0.13 0.46 22 210 5.1 0.16 0.54 23 176 2.5 0.16 0.53 24 320 2.50.12 0.43 25 206 0.95 0.20 0.61

Examples 26-30

Microporous PVDF films were prepared as in Examples 8-9 and whereby theTRIACETIN diluent was washed out of the membranes using a water washbath immediately after the chrome roll. The microporous PVDF membranesbecame hydrophobic after removal of the diluent, making it difficult tofill the membrane pores with ion conducting solutions to prepare themembranes for use as a PEM. To decrease the hydrophobicity of themembrane, a solution of n-propanol and glycerol, ranging fromapproximately 65:35 to 75:25, was used to pre-wet the membrane using thefollowing procedure. Two glass plates were cleaned with a 50:50 solutionof isopropanol and water. A piece of microporous film was taped to oneof the cleaned plates. The plates were then placed end to end and then,using an applicator gage, the “blank” glass plate was coated with then-propanol/glycerol solution and the film carefully laid into thesolution. A rubber roller was used to squeegee the excess solution offthe film using broad strokes. The second plate was quickly coated withthe NAFION 1000 ionomer solution using the cleaned/wiped applicatorgage. The pre-wetted film was then laid into the ionomer solution,making sure to avoid trapping bubbles or creating wrinkles in the film.The ionomer solution was then allowed to diffuse through the membranefor 10-30 seconds. The top of the membrane was then coated with thedesired thickness ionomer solution using the applicator gage. Themembrane was then dried at 90° C. for 10 minutes and annealed at 160° C.for an additional 10 minutes. The resulting ion conductive membrane(ICM) was subsequently assembled into a membrane electrode assembly fortesting via the above described test methods. Porosity and electricalproperties of the ICMs are shown in TABLES 5 and 6 below. TABLE 5 % P.B.60 % Nucleating TRIACETIN Gurley B.P. Pore Thickness Example Agentdiluent sec/50 cc Size, μm (μm) 26 0.25 58 3.5 1.54 30.5 27 0.5 58 4.31.29 30.5 28 0.5 58 5.1 1.21 30.5 29 0.5 58 6.5 0.93 25.4 30 0.5 58 4.31.29 38.1

TABLE 6 H₂ Xover H₂ pump (10/0 psi) Perf @.8 V Perf @.6 V ExamplemOhm-cm² mA/cm² A/cm² A/cm² 26 122 5.07 0.22 0.70 27 115 1.18 0.24 0.7428 141 1.13 0.21 0.67 29 128 1.18 0.24 0.70 30 139 1.21 0.21 0.73

The PVDF ICMs of Examples 20-24 were also subjected to puncture tests todemonstrate their improved strength as compared to a conventional ICMprepared from a NAFION 1000 solution that was cast and then dried. Fivedifferent NAFION control films were produced and measured. The puncturestrengths of the membranes were normalized by dividing the strength bythe thickness to show that on a per-unit thickness basis, the membranesof the current invention were stronger than a conventional ICM. TABLE 7Normalized Puncture Strength Puncture Strength Example Thickness (μm)peak load (grams) (gms/μm) 20 17.8 16.9 0.95 21 17.8 15.9 0.89 22 15.212.9 0.85 23 20.3 16.7 0.82 24 12.7 13.1 1.03 Control 2 30.5 16.9 0.55Control 3 30.5 15.9 0.52 Control 4 26.5 12.9 0.49 Control 5 31.8 16.70.53 Control 6 26.2 13.2 0.50

The overall puncture performance was comparable to conventional densepolymer electrolyte PEMs, even though the composite PVDF proton exchangemembranes had thicknesses less than that of the dense membranes. Themicroporous PVDF films of the present invention were successfullyimbibed to function as PEMs in MEAs.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

All patents, applications, and publications mentioned herein areincorporated herein by reference.

1. An ion conductive membrane comprising: a) a shaped articlecomprising: polyvinylidene fluoride or copolymers thereof, a sufficientquantity of nucleating agent to initiate crystallization of thepolyvinylidene fluoride or copolymers thereof at a significantly greaternumber of crystallization sites as compared to crystallization withoutthe nucleating agent, and wherein the shaped article has been orientedin at least one direction at a stretch ratio of at least approximately1.1 to 1.0 to provide a network of micropores wherein the micropore sizeis greater than approximately 0.4 microns, and the shaped article has athickness less than approximately 1.5 mils and a Gurley less thanapproximately 10 sec/50 cc; and b) a sufficient quantity of ionconducting electrolyte filling the micropores to allow the membrane tofunction as a ion conductive membrane.
 2. An ion conductive membrane ofclaim 1, wherein the sufficient quantity of nucleating agent is betweenapproximately 0.2 percent to approximately 2.5 percent by weight ofpolyvinylidene fluoride or copolymers thereof.
 3. An ion conductivemembrane of claim 2 wherein the nucleating agent is selected from thegroup consisting of Pigment Blue 60, Pigment Red 179, Pigment Violet5:1, Vat Yellow 2, Pigment Yellow 24, and polytetrafluoroethylene.
 4. Anion conductive membrane of claim 1, wherein the polyvinylidene fluorideor copolymers thereof are semicrystalline and have melt flow indicesbetween approximately 0.13 to approximately 6.0.
 5. An ion conductivemembrane of claim 1, wherein the shaped article is biaxially oriented ata stretch ratio of 1.1 to 1.0.
 6. An ion conductive membrane of claim 1,wherein a sufficient quantity of ion conducting electrolyte is thevolume of ion conducting electrolyte sufficient to fill at leastapproximately 95 to 100% or more of the pore volume of the membrane. 7.A membrane electrode assembly comprising the ion conductive membrane ofclaim
 1. 8. An electrochemical device comprising the membrane electrodeassembly of claim
 7. 9. A fuel cell comprising the membrane electrodeassembly of claim 7.